Your comfort over the winter depends on the right furnace, but the complexities of furnace technology can make it hard to know which model to choose. Air flow configuration – upflow, downflow, and horizontal flow – is just one factor you might need to consider.
If designed and installed correctly, any of these systems can keep your home comfortable. The right configuration for your home depends largely on where the furnace will be located.
Upflow Furnaces: Ideal for the Basement
In an upflow furnace, cool air from your house enters either at the bottom or top of the air handler, depending on the furnace model, and heated air flows out the top. The heated air then moves into the supply ductwork and off to your rooms. Upflow furnaces are most often installed in basements and, less commonly, in crawl spaces.
In these locations, the ductwork is above the furnace, so the upflow configuration takes advantage of the laws of thermodynamics. Because warm air rises, thanks to the law of convection, air from an upflow furnace in the basement naturally rises toward the rooms above it. It needs less help from the blower fan to move into the ducts, so you save some energy.
On the other hand, if an upflow furnace were installed in an attic, you’d be wasting energy. The heated air would naturally rise toward your roof, so the blower fan would have to use more energy trying to push it down toward your rooms.
Upflow furnaces come in “highboy” and “lowboy” models. Highboys are typically no higher than 6 feet and designed with the blower fan under the heat exchanger. Lowboys are a maximum of 4 feet high with the blower fan behind or beside the heat exchanger to save vertical space. While shorter, lowboys tend to be deeper front to back than highboys. Both can perform equally well, but lowboys are more practical for low-ceiling basements.
An upflow furnace in the basement offers several advantages beyond energy efficiency. The airflow will come from floor registers, rather than the ceiling registers common in attic installations. Warm air from below tends to be more comfortable and is less likely to pool in one place compared to warm air from above. Installation is also simpler. You won’t need to upgrade the flooring as you would with an attic furnace.
Downflow Furnaces: A More Efficient Choice for the Attic
Also known as “downdraft” or “counterflow” furnaces, downflow furnaces are designed to bring in cool air through the top of the air handler and release heated air from the bottom. These models are usually installed in the attic or, less often, in a closest on the home’s main floor or in the garage. They’re the most popular choice for homes with a slab foundation or a crawl space rather than a basement.
The benefit of these furnaces is that they can be installed in any type of home. If you don’t have a basement or crawl space, an upflow furnace just isn’t a practical choice. You aren’t necessarily stuck with ceiling registers, either. In many homes, downflow furnaces can also work with floor registers.
On the downside, these furnaces work against the natural tendency of warm air to flow upward, so they’re less efficient than upflow models. Even though the furnace releases air at the bottom, that air will still want to rise. What’s more, if you want your furnace in the attic, you’ll need to have the subflooring reinforced to support the extra weight and a separate subbase installed as a fire-resistant barrier between the furnace and flooring. That’s a minor construction job in itself.
Horizontal Flow Furnaces: Space-saving and Flexible
A horizontal furnace is designed with the components in a horizontal row, so the unit looks like it’s lying on its side. Cool air flows in on one side, and heated air flows out the other. These furnaces are designed as either “horizontal left” or “horizontal right,” referring to the side the heated air comes from. This gives your HVAC installer more options for designing the most efficient setup for your home.
Horizontal flow systems are most often installed in attics, and crawl spaces where space is limited due to a low ceiling and a standard upflow or downflow furnace just won’t fit.
Upflow/horizontal and downflow/horizontal models are also available. These offer two possible configurations. For example, an upflow/horizontal model can be configured as an upflow furnace or as a horizontal flow furnace, depending on your home’s needs.
For maximum flexibility, there are multi-positional or “multipoise” furnaces. Your installer can configure one of these as upflow, downflow or either left- or right-side horizontal flow.
A furnace with the right air flow configuration for your home’s design will keep you more comfortable and help control your heating costs. If your home has a basement, chances are your installer will recommend an upflow furnace or, if the ceiling is very low, a horizontal flow model. No basement? Then a downflow model is most likely the way to go, but you have a few choices for where it’s installed.
If you’re not sure, ask your HVAC installer. No two homes are exactly alike, and your installer can explain the nuances of why a particular air flow configuration is the best option for your home.